I was really interested by the figure 3 at the chapter 12.4.
I thought about that before, how to sync 2 differents Meteor servers for some types of publications that requires performance or no persistence, for example notifications or counts or chat.
The solution is to use redis between the Meteor servers as Socket.io does for syncing the different sockets (with its simple adapter). But I saw no where how to use this strategy in Meteor and I think it is a really important point to explain, and if you do, it will be the only book to explain it (I have Discover Meteor and even Bulletproof Meteor which is supposed to cover that but doesn't).

Puuh, as much as I'd love to bring this topic into Meteor in Action I think it would go beyond what we can fit into it. This is quite an advanced technique but I am certain this could be handled in a blog post in the not-so-distant future